There has been significant progress in recent years in performance and diversification of various electronic apparatuses such as mobile phones and portable terminals. These apparatuses use optically transparent touch panels in a manner that they are mounted to front faces of display devices such as liquid crystal displays. When making selection of any of functions of an electronic apparatus, a user operates the touch panel by pressing it with a finger, a pen or the like while visually checking a screen of the display device disposed to the backside of this see-through touch panel. There is therefore a demand for electronic apparatuses having easy-to-see screens of display devices behind touch panels, in addition to good operability.
The accompanying figures show touch panels illustrated in a scale enlarged only in the dimension of thickness to make their structures readily understandable.
FIG. 3 is a sectional view of a touch panel of the prior art. Touch panel 10 comprises a film-like transparent upper substrate 1, and transparent lower substrate 2 made of a pane of glass or the like material. Upper substrate 1 has transparent upper conductive layer 3 of indium-tin oxide or the like material formed on main surface 1a, and lower substrate 2 has lower conductive layer 4 formed on its main surface 2a through antireflection layer 5.
Antireflection layer 5 is thus disposed between lower conductive layer 4 and lower substrate 2. Antireflection layer 5 comprises low-refraction layer 5A made of silicon oxide, magnesium fluoride or the like material having a smaller refractive index than that of lower conductive layer 4, and high-refraction layer 5B made of niobium oxide, titanium oxide or the like material having nearly same refractive index as lower conductive layer 4.
Lower conductive layer 4 has a plurality of dot spacers (not shown in the figure) formed of an insulation resin at regular intervals on one of the main surfaces facing spacer 6. Upper conductive layer 3 is provided with a pair of upper electrodes (not shown) formed at two side ends thereof, and lower conductive layer 4 is also provided with a pair of lower electrodes (not shown) at two side ends thereof in a direction orthogonal to the upper electrodes.
Spacer 6 has a frame-like shape, and it bonds between upper substrate 1 and lower substrate 2 along their peripheries with adhesive layers (not shown) formed on both of its upper and lower surfaces. In touch panel 10 of this kind, upper conductive layer 3 and lower conductive layer 4 are arranged in a manner to face each other with a predetermined space.
Touch panel 10 constructed as above is placed on a front face of a liquid crystal display or the like device and mounted to an electronic apparatus of any kind. The pairs of not-illustrated upper and lower electrodes are then connected to an electronic circuit (not shown) employed in the electronic apparatus of such kind.
In this touch panel 10, when a user presses main surface 1b of upper substrate 1 with a finger, a pen or the like while visually checking a screen of the liquid crystal display or the like device behind the touch panel, upper substrate 1 bends in a manner that upper conductive layer 3 at the pressed portion comes into contact with lower conductive layer 4.
A voltage is then impressed from the not-illustrated electronic circuit to the upper electrodes and the lower electrodes in a consecutive manner, so that the electronic circuit detects the portion being pressed according to a voltage ratio between these electrodes, and switches a variety of functions provided in the electronic apparatus.
When touch panel 10 is used in a bright environment, especially outdoors or under fluorescent lamps, the external light such as sunlight and lamplight passing through upper substrate 1 and upper conductive layer 3 is reflected in lower conductive layer 4 provided on main surface 2a of lower substrate 2, and this gives rise to a problem of making the screen in the liquid crystal display device difficult to see since it is located behind the touch panel. Antireflection layer 5 is therefore placed between lower conductive layer 4 and lower substrate 2 for the purpose of preventing reflection of the external light.
In other words, touch panel 10 of the prior art comprises low-refraction layer 5A of a comparatively small refractive index and high-refraction layer 5B of a comparatively large refractive index, both of which are laminated on main surface 4b of lower conductive layer 4 of generally the same refractive index as high-refraction layer 5B. Because of this structure, the external light hitting upon main surface 4a of lower conductive layer 4 is allowed to pass easily through the interior of lower conductive layer 4, low-refraction layer 5A, and high-refraction layer 5B. Since this results in a reduction of the external light reflected in main surface 4a of lower conductive layer 4 by the amount allowed to pass therethrough, the structure here provides the user with an easy-to-see screen of the liquid crystal display device behind touch panel 10 by virtue of this reduction of the reflected external light.
One example of the prior art techniques relevant to the present invention is illustrated in International Publication, No. 02/029830.
In the conventional touch panel 10 described above, however, high-refraction layer 5B and low-refraction layer 5A are laminated to form antireflection layer 5 on main surface 2a of lower substrate 2 made of glass or the like material. High-refraction layer 5B formed of niobium oxide, titanium oxide or the like has weak adhesion to lower substrate 2 of the glass, which leaves a problem that antireflection layer 5 is liable to separate from lower substrate 2 if touch panel 10 is warped due to an excessive force applied to it during manufacturing, transporting or mounting to an electronic apparatus.
The present invention is contrived to overcome the above problem of the prior art technique, and to provide a touch panel having a simple structure and easy to operate while preventing separation of the antireflection layer from the lower substrate.